As a fixing device provided in a photocopier or a printer, there is a type of the fixing device including an endless belt, a ceramic heater which comes into contact with the inner surface of the endless belt, and a pressure roller which forms a fixing nip portion together with the ceramic heater via the endless belt. When small-size sheets are continuously printed by an image forming apparatus provided with this fixing device, a phenomenon (the temperature rise of a non-sheet feeding portion) in which the temperature gradually rises in an area through which no sheet passes in the longitudinal direction of the fixing nip portion occurs. If the temperature of the non-sheet feeding portion excessively rises, parts in the device are damaged, or toner offsets at a high temperature causes in the area corresponding to the non-sheet feeding portion of the small-size sheets when a large-size sheets are printed in a state where the temperature rises at the non-sheet feeding portion.
As one of means to suppress the temperature rise of the non-sheet feeding portion, it is considered that a heat generating resistor on a ceramic substrate is made of a material having negative resistance temperature characteristics. Even if the temperature of the non-sheet feeding portion rises, the resistance value of the heat generating resistor of the non-sheet feeding portion lowers. Therefore, it is considered that even when a current flows through the heat generating resistor of the non-sheet feeding portion, the heat generation of the non-sheet feeding portion is suppressed. In the negative resistance temperature characteristics, when the temperature rises, the resistance lowers. Hereinafter, the characteristics will be referred to as a negative temperature coefficient (NTC). Conversely, it is suggested that the heat generating resistor is made of a material having positive resistance temperature characteristics. It is considered that when the temperature of the non-sheet feeding portion rises, the resistance value of the heat generating resistor of the non-sheet feeding portion rises, and the current flowing through the heat generating resistor of the non-sheet feeding portion is suppressed to inhibit the heat generation of the non-sheet feeding portion. In the positive resistance temperature characteristics, when the temperature rises, the resistance rises. Hereinafter, the characteristics will be referred to as a positive temperature coefficient (PTC).
However, the material having the NTC usually has a very high volume resistance. It is very difficult to set the total resistance of the heat generating resistors formed in one heater to a range usable with a commercial power supply. Conversely, the material having the PTC has a very low volume resistance. In the same manner as in the material having the NTC, it is very difficult to set the total resistance of the heat generating resistors in the heater to the range usable with the commercial power supply.
To solve such a problem, the heat generating resistors of the PTC formed on the ceramic substrate are divided by a plurality of heat blocks in the longitudinal direction of the heater. In each of the heat blocks, two conductive members are arranged at both ends of the block in the lateral direction of the substrate so that the current flows through the block in the lateral direction of the heater (the conveyance direction of a recording sheet). Furthermore, Japanese Patent Application Laid-Open No. 2005-209493 discloses the plurality of heat blocks electrically connected in series. According to such a constitution, even when the heat generating resistor of the PTC is used, the total resistance of the heater can easily be set to the range usable with the commercial power supply. Moreover, this document also discloses that a plurality of heat generating resistors is electrically connected in parallel between two conductive members to form the heat block.